1. Field of the Invention
The present invention generally relates to a MOS test structure, a method for forming a MOS test structure and a method for performing a wafer acceptance test (WAT). In particular, the present invention generally relates to a MOS test structure with a conductive material filling a test via, a method for forming the MOS test structure and a method for performing a wafer acceptance test (WAT) using the MOS test structure.
2. Description of the Prior Art
A trench gate MOS is one of the MOS structures used in semiconductor devices. To ensure manufacturing processes meet pre-determined standards, some electrical properties of the unfinished semiconductor devices have to be tested. For example, when a trench gate is formed in a doping well and in a doping region which are disposed on a substrate and on an epitaxial layer, a wafer acceptance test (WAT) should be carried out before a back side polishing from one side of a wafer.
If the wafer acceptance test (WAT) is directly carried out on the outmost doping well layer and the outmost substrate, the test result is an overall result of the doping well, the doping region, the epitaxial layer and the substrate. In other words, the test results of the epitaxial layer and the substrate cannot be obtained without the interference of the doping well and the doping region, which is not acceptable.
One test structure has been proposed to obtain the data of the epitaxial layer and the substrate without the interference of the doping well and the doping region. An additional heavily doped well is designed to be disposed adjacent to the doping well, the doping region, the epitaxial layer and the substrate, electrically connected to the doping well, the doping region, the epitaxial layer and the substrate, and in direct contact with the doping well, the doping region, the epitaxial layer and the substrate. A signal is applied on the drain side to pick up the signal from the heavily doped well to obtain the data of the epitaxial layer and the substrate without the interference of the doping well and the doping region. However, such additional heavily doped well is specially designed and occupies a considerable area. Also, the data is not completely independent of the interference by the doping well and the doping region since the additional heavily doped well is still electrically connected to the doping well and the doping region.